A requirement increasingly imposed on power converters fed by utility supplies is that the Power Factor exceed a lower limit, typically 0.9. This implies that the voltage supplied and current drawn are substantially in-phase with sinusoidal current, resulting as shown by the sinusoidal rectified line current in the lower part of FIG. 1(a) where there is power factor correction. In contrast the upper part illustrates what the rectified line current might typically look like without power factor correction. As a result, Power Factor Correction (“PFC”) is becoming a key functional requirement of power converters.
It will be appreciated that there are a variety of different architectural approaches to PFC. Some of these architectural options for power factor correction are as shown in the three general options of FIG. 1(b). The nature of each of these options will be understood by those skilled in the art. Each of the options takes a line (mains) voltage (VLine) and converts it through a series of stages to provide power to a load 10. In the first option, the line voltage is rectified through a rectification stage 12. The rectified voltage from the rectification stage acts as an input to a pre-regulator stage 14, which in turn charges a bulk capacitance 16. A high frequency transformer stage 18 is then provided to convert the voltage on the bulk capacitance to the desired voltage for the load 10. It will be appreciated that the high frequency transformer section also isolates the mains side from the low voltage DC side on the output. It will be appreciated that high-frequency is relative to the incoming main frequency of 50 Hz and may for example be of the order of 10-20 kHz.
In the second option, the rectification and pre-regulator functions are combined together in stage 20. This type of arrangement is generally referred to as a “Bridgeless” approach. In the third approach, which is a bus converter approach, the line voltage is rectified by a rectifier stage 12 before being switched by a high frequency transformer stage (bus converter) 22, which is followed by a pre-regulator stage 24, bulk capacitance and then a post-regulator stage 26.
There is of course the opportunity for successive functional blocks to be integrated to perform multiple roles, which may be attractive in terms of reducing cost particularly at lower power levels. For example a single stage may perform both isolation and power factor correction.
As shown, Power Factor Correction is typically implemented by a boost pre-regulator 20, or in some cases by modifying a flyback topology to widen the conduction angle to meet norms. The buck pre-regulator can also have application in some roles, but has the significant disadvantage of only drawing current when the input voltage is above the output voltage of the buck stage.
The operating conditions of the power factor correction functional block vary significantly. Typically an input range of 90V root-mean-square (“RMS”) to 264V RMS is required. Within a sinusoidal line cycle, the instantaneous power handled at the peak is twice the RMS power, thus imposing further variability on circuit operation.
The conventional design approach taken when designing a converter is associated with removing this variability as early as possible in the conversion stage, such that a small number of stages, indeed generally just one, has to deal with the variability in operating conditions, and “downstream” stages can work under the near constant operating conditions associated with high efficiency.
The input stage of a power converter also has challenges in terms of managing surges, where high voltages can be applied to the line for several tens of microseconds, in terms of managing inrush currents to a large electrolytic capacitor as used in boost power factor correction circuits and in managing noise that may be transferred to the AC line. The typical approach also uses a bridge rectifier, with the attendant voltage drop of approximately 1.2V-1.6V that accounts for loss of typically 1.5% at low line.
A particular challenge in the context of improving power density in power converters fed from the AC line relates to the switching frequency of the power factor correction stage. This stage operates necessarily from line voltage under the wide range of operating conditions as discussed, and the “conventional” stages are challenged in terms of implementing soft-switching as needed for high efficiency.
Some boundary-mode operation holds potential—but this is difficult to manage across the full range of operating conditions. Gallium Nitride devices are associated with lower parasitics in terms of capacitance for given switch on-resistance, and such devices can be used in converters for power factor correction at higher frequencies. The potential attractions of efficient high-frequency operation include significant reduction of magnetic elements in the main converter stage, as well as reduction of size of magnetics and capacitance in the EMI filter.
The present application is directed to techniques which assist in getting to this position—i.e. having small passive components consistent with high efficiency in implementing power factor correction.